My mixer is out of inputs 
I have things I need to add, but my 8 channel MIDIBOX is eating most of the available space. What is the best approach to get a clean stereo output from all 8 SID chips?
The MIB-6582 uses a passive mixer, a design which I melded together with the CMoy audio amp design. You can find my (untested) work on the [=sidr8tr]wiki](mb-sidr8tr:start [MIDIbox]). It probably works as it should but I haven’t reached that phase of my synth build yet so use at your own risk. You need a bipolar supply, but I was going to have one anyway in my design since I was wanting to use analog filters as well.
For me the passive mixing in the mb6582 works very fine, i don´t even need amplification when going into an ordinary mixer with 2 channels.